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Appendix G ·eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee Glossary Anisotropy: See the definition of Isotropy. Batch: The word "batch" in glass technology is used conventionally in two ways: batch materials are precursor materials that, when heated in the melter, react to form the molten glass; a batch process is one that operates on an intermittent or interrupted basis (i.e., is not continuous) and in which the processing vessel (e.g., the melter) is emptied before being replenished. Becquerel: The international unit of radioactivity, equal to one disinte- gration or other nuclear transformation per second. The historical unit of radioactivity is the curie, equivalent to 3.7 x 104° bec- querels. Bismuth phosphate process: This process separated plutonium from uranium and other radionuclides in spent fuel. It operated on an industrial scale at the Hanford Site from 1944 to 1956. Following dissolution of the aluminum cladding in hot sodium hydroxide solution, the uranium metal fuel slugs were dissolved in nitric acid. The dissolved fuel solution was treated with several succes- sive cycles of precipitation to separate plutonium from uranium and other elements. The first involved inducing precipitation with bismuth phosphate by way of co-precipitation with lanthanum fluoride. When sufficiently concentrated, the plutonium was sep- arated from the lanthanum fluoride carrier. The acidic HEW from this process contained uranium, fission products, and a significant fraction of the plutonium. It was made alkaline with sodium hydroxide prior to storage in tank farms. Calcination: Decomposition of a material to a powder, usually involv- ing an oxidizing atmosphere at an elevated temperature, for the purpose of changing its properties (e.g., to dehydrate or to cause H ~ G H - L E V E E VV A 5 T E ~ A ~ 126
thermal decomposition of n itrates, n itrites, oxal ates, formates, and similar anions). Calcine: General term for the powder generated by calcination. For the INEEL HLW program, it is a specific product from a fluidized bed calciner. Curie: Measure of radioactivity equal to 3.7 x 104° disintegrations per second, an historical unit representing approximately the radio- activity of 1 gram of radium-226. The Systeme International or Sl unit for radioactivity is the becquerel, 1 disintegration per second. Decontamination factor: The (dimensionless) ratio of the concentration of a species of interest in an original, input stream (to a process) to the concentration in a final, output stream. The typical quanti- ty used for this ratio is the radioactivity per unit volume or mass of carrier (e.g., solvent) material. Electrode: The conducting material used in electric melters to pass an alternating current through the preheated glass batch so as to cause internal resistive heating and melt formation. Electric melters for commercial glass production typically employ molyb- denum or tin oxide electrodes. The Joule-heated melters at DWPF and WVDP use Inconel-690@ alloy electrodes. Inconel-690@ is a registered trademark of Inco Alloys International Inc., Huntington, West Virginia. Elution: Flushing of a solvent through a sorbent material to eliminate the adsorbed component. Environmental Assessment (EA) glass: Alkali borosilicate glass devel- oped at the Savannah River Site, used in the Environmental Assessment of the DWPF, and specified in the SRS Waste Acceptance Product Specification (WARS) as a benchmark for comparison, based on the durability test. The WARS requires that HLW glasses be more durable (in the Product Consistency Test leach test analyses) than EA glass. Foaming: Resu It of redox reactions and the breakdown of an ions (nitrates, carbonates, etc.) that generate gas during melting. Excessive foam can form a physical and thermal barrier between the cold cap and the melt pool and can lead ultimately to melter shutdown. Frit: Premelted glass that has been crushed to a fine powder and used for further ceramic processes (e.g. ceramic glaze formulation). The frits used for HLW vitrification are premelted borosilicate glass powders that are added to the waste and fed to the melter. A p p e n d i x G
Glass-in-glass phase separation: Phenomenon that occurs when a melt separates into two or more amorphous phases during cooling. Depending on the mechanism and the relative volumes of the separated phases, the morphology may vary from encapsulated droplets in a continuous matrix to completely continuous inter- dispersed phase regions. The concern with borosilicate systems is that phase separation could lead to the formation of borate-rich gl ass regions of poor d u rabi I ity. Gray: The unit of absorbed dose equal to the absorption of one joule per kilogram of absorbing material. High-level waste: High-level waste can be (1 ) irradiated reactor fuel; (2) liquid waste resulting from the operation of the first-cycle solvent extraction system, or equivalent, and the concentrated waste from subsequent extraction cycles, or equivalent, in a facility for repro- cessing irradiated reactor fuel; and (3) solids into which such liq- uid wastes have been converted (Title 10 CFR Part 60.21. In loose terms, HLW is the waste containing fission products and actinides that results from the reprocessing of spent nuclear fuel and requires permanent isolation in a geologic repository. On-exchange: Chemical reaction in which mobile hydrated ions of a solid are exchanged, equivalent for equivalent, for ions of like charge in solution. One or more of the ionic species in solution are selectively absorbed on a solid and retained. Isotropy: The feature of having equal physical properties (refractive index, thermal expansivity, elastic constants, etc.) in different crystallographic directions. Cubic crystals are rarely completely isotropic, but are considerably more so than other crystal systems (tetragonal, hexagonal, orthorhombic, monoclinic or triclinic) with lower symmetry, which are usually strongly anisotropic. Leaching: Chemical dissolution of a solid in a liquid, which may be congruent (i.e. the solid dissolves uniformly) or incongruent LO ' ' ~ ' LO tcnemical components at the sol id are dissolved at ditterent rates). Borosilicate waste glasses are required to be leach resistant with respect to aqueous liquids (deionized water or groundwater) and usually exhibit incongruent leaching, with the most mobile ions being the alkali ions (including cesium) and boron. Low-level waste: Any radioactive waste that is not spent fuel or high- level waste. Methathesis: Double decomposition chemical reaction of the type AB + CD ~ AD + CB that is driven by the law of mass action when there is an excess of one ion. H ~ G H - L E V E E W A S T E
Mixed waste (MW): Waste that contains both chemically hazardous constituents regulated under RCRA and radioactive materials reg- ulated under the Atomic Energy Act. Off-gas emissions: Gaseous and particulate material that is vaporized from the surface of the melt pool in a glass melter. Partitioning: Process that divides an input stream into two or more out- put streams. A mixture of solids can be partitioned based on dif- ferences in material properties. A liquid phase with two or more chemical species can be partitioned using selective media to seg- regate the species based on their differences in chemical reactiv- ity. Phenomenological model: A multi-component model with predictive power that combines a series of mathematical descriptions of the individual phenomena involved. Some of these processes occur simultaneously, others occur sequentially. In the case of waste- form leaching, these will probably include some or all of the fol- lowing: · H+ or H3O+ diffusion into the glass surface; · diffusion of leachable species (particularly alkali ions) out of the glass surface; · subsequent diffusion of ions across the external leached layer; · dissolution of the leached layer itself (i.e., matrix dissolution); · processes (e.g., fractu ri ng) that alter the gl ass su rface-area/sol u- tion-vol ume ratio (SA/V ratio); · precipitation reactions on the glass surface from interaction with the chemical components of groundwater; · groundwater radiolysis and its effect on wasteform/groundwa- ter interactions; · internal radiation effects on the structure and durability of the wasteform. Product ConsistencyTest (PCT): The product consistency test is a leach test that evaluates the chemical durability of glasses by measuring the concentrations of the chemical species (boron, sodium, and lithium) released from a crushed glass to a test solution. The PCT is one of the standard tests of the American Society for Testing and Materials (C1 285-97~. PUREX Process: This process purified and separated both uranium and plutonium from spent fuel. It was used at all four HEW sites and
first used at Hanford in 1 952. The first step consists of removing the cladding of spent fuel (fuel preparation) to expose the fuel rods. In the second step, spent fuel is dissolved in nitric acid. The third and key step is the separation and recovery of uranium and plutonium from other fission products, which is done in a contin- uous counter/current solvent extraction process using tri-N-butyl phosphate in a hydrocarbon diluent (such as kerosene). Plutonium and uranium remain dissolved in the aqueous nitric solution as nitrates. This step produced the majority of HEW, con- sisting of a nitric acid solution of fission products and impurities. The fourth step consists of separating plutonium from uranium followed by conversion to solid oxide or metal. Except at INEEL, the waste was neutralized with sodium hydroxide prior to storage in the tank farms. A major advantage of the PUREX process over previous processes used (see B ismuth phosphate process and REDOX process) is that nitric acid is the only salting agent used in the aqueous phase; therefore HEW has a lower salt content than waste from the other processes. Other advantages of PUREX are reduction of the waste volume, greater flexibility in process conditions, reduction of fire and other hazards, and reduction of costs. Processes based on PUREX have been adopted for nearly al I fuel reprocessing throughout the world. Rad: Radiation absorbed dose; the basic unit of absorbed dose equal to the absorption of 0.01 joules per kilogram of absorbing material. Radiolysis: The chemical modification or decomposition induced by ionizing or near-ionizing radiation. In the case of borosilicate glasses, radiolysis can lead also to atomic displacement. Raffinate: Aqueous solution remaining after a metal has been extracted by the solvent. REDOX process: The REDOX process purified and separated both ura- nium and plutonium from spent fuel. It was operated at Hanford from 1 951 to 1 967. Th is was a conti n uous cou nter/cu rrent solvent extraction process using methyl isobutyl ketone (hexone) as the solvent. With hexone, it was necessary to add aluminum nitrate to the aqueous phase to achieve the high ionic strength required for effective extraction of the uranium and plutonium. This process had major advantages over the bismuth phosphate process: it reduced the waste volume, provided for nearly com- plete recovery of both uranium and plutonium, and allowed con- tinuous operation, which favored higher production rates. One disadvantage was that large amounts of aluminum nitrate had to H ~ G H - L E V E E W A S T E
be added. The acidic waste, containing fission products, alu- minum nitrate, and impurities, but very little uranium or plutoni- um, was made alkaline with sodium hydroxide prior to storage in the tank farms. Refractories for glass melters: The high-temperature materials used for thermal insulation and for containing molten glass. The glass con- tact refractories are selected for high corrosion resistance, while the backing refractories are chosen for their insulating properties. Refractories used in electric melters must also exhibit high elec- trical resistivities. The glass contact refractories used in the Joule- heated melters at DWPF and WVDP are bricks of chrome-rich Monofrax K3@, a fusion-cast refractory material that shows excel- lent corrosion resistance, but with a trend toward decreasing elec- trical resistivity with increasing temperature that can be problem- atic much above 11 50°C. Monofrax is a registered trademark of the Carboru nd u m Company, Fal caner, N ew York. Repository: A place for the disposal of immobilized high-level waste to isolate it from the environment. Reprocessing: Recovery of fissile and fertile material for further use from spent fuel by chemical separation of uranium and plutoni- um from other transuranic elements and fission products. Selected fission products may also be recovered. This operation results in the generation of HEW. Saltcake: concentrated waste i n the form of crystal I ized salts resu Iti ng from the evaporation of liquid high-level waste. Saltstone: Low-level wasteform material used at the Savannah River Site, consisting of a cement-based material in which the low-level waste is either dissolved or dispersed. Sludge: Semi-solid, viscous, amorphous material that is substantially insoluble. The sludge in HLW tanks is formed by gravity settling of insoluble solids (usually hydroxides) on the tank floor after waste neutral ization with sodium hydroxide. Slurry: Suspension of insoluble (usually finely powdered) materials in a liquid medium. Solvent extraction: Separation of materials of different chemical types and solubilities by selective solvent action. This term is a general process for separating one or more chemicals solubilized in one solvent by the use of a second solvent that is (1 ) insoluble in the first solvent and (2) selective in its ability to bind one or more of A p p e n d i x ~
the chemical constituents. After co-mixing and agitation of the two solvents, they separate, with the second having extracted the chemical species of interest. Source term: The initial amount of radioactivity used to calculate expo- sure and doses to various receptor groups. Spent nuclear fuel: Fuel that has been withdrawn from a nuclear reac- tor following irradiation, the constituent elements of which have not been separated by reprocessing. Spinel: Family of cubic-structured ceramic oxides of general formula A2+ B23+O4. The spinel compounds that can form precipitates in borosilicate glass during high-level waste vitrification are pre- dominantly oxide compounds of iron, nickel, and chromium. Transuranic waste: Waste contaminated with transuranic elements with half-lives greater than 20 years, in concentrations greater than 100 nanocuries per gram. Waste loading: Weight fraction of waste immobilized in the waste form (e.g., borosilicate glass). Some ambiguity has been caused by the use of the term to describe either the weight fraction of the origi- nal waste components (salts, hydroxides, etc.) in the waste form or the amount of equivalent oxides that will be formed by thermal decomposition during waste form production. A further ambigui- ty arises from use of the term at different HEW sites to either include or exclude nonradioactive waste components (e.g., Na, Al, Si) that constitute part of the final waste form matrix. H ~ G H - L E V E E W A S T E 132
